In the past decade, applications of spectroscopy and microspectroscopy have greatly advanced into areas of clinical study. The potential of various spectroscopic techniques for screening and disease diagnosis in clinical settings has been investigated.
For instance, infrared microspectroscopy has been used in the study of biological samples. As is well known, this technique involves illuminating the sample being studied with infrared light, and collecting the infrared light from a selected microscopic region of the sample to derive the absorption spectrum of that region. The measured infrared spectra from different regions of the sample are analyzed to identify cell types or anomalies. The results of the spectroscopic measurements are typically compared to the results of a study by a pathologist on a separate sample from the same source for classification.
Recently, Fourier Transform Infrared (FT-IR) spectroscopic imaging microscopy has been developed into a very powerful analytical technique. This technique uses a focal-plane array detector attached to an FT-IR microscope to collect infrared images of an area of interest on the sample. The focal-plane array detector includes an array (e.g., 64.times.64) of pixels, each capable of independently detecting the intensity of infrared light impinging thereupon. A significant advantage of this technique as compared to more conventional infrared microspectroscopy is the parallel infrared detection of a relatively large number of pixels, which eliminates the need of point-by-point mapping of the sample. This parallel detection significantly reduces the time required to collect infrared spectra of a given sample.
The FT-IR imaging microscopy, however, is not readily applicable to biological samples conventionally prepared for pathological studies. The FT-IR imaging microspectroscopy is typically performed in a transmission mode. For that purpose, the sample is fixed on a window made of an infrared-transparent material, such as BaF.sub.2 or CaF.sub.2. Infrared light for illuminating the sample is directed through the window to the sample, and the infrared light passed through the sample is collected for spectral analysis. In contrast, a pathologist uses visible microscopy to analyze a biological sample. A biological sample for pathological studies is typically a thin section fixed on a glass slide and stained. Glass slides, which are transparent to visible light, are strongly absorptive in the mid-infrared range that is important for spectral analysis. As a result, a biological sample prepared for conventional pathological studies cannot be used for FT-IR imaging.
It is possible to mount a biological sample on an infrared-transparent window so that it can be studied with FT-IR spectroscopy. This approach is not preferred, however, for several reasons. The infrared-transparent window, which is typically made of a certain type of salt, may react with the biological sample supported thereon. Moreover, infrared-transparent windows are very expensive and more difficult to handle than conventional glass slides. Furthermore, perhaps the most significant drawback of this approach is that the sample prepared for the infrared study is damaged in the sense that it cannot later be recovered for examination by a pathologist to verify the diagnoses based on the infrared measurements or to perform any other pathologic studies. The use of different samples prepared in different ways, albeit from the same source, for infrared spectroscopic and conventional pathological studies inevitably introduces some unreliability in the comparison of the results of the two studies.